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1500 questions
52
votes
4 answers
What's the real fundamental definition of energy?
Some physical quantities like position, velocity, momentum and force, have precise definition even on basic textbooks, however energy is a little confusing for me. My point here is: using our intuition we know what momentum should be and also we…
Gold
- 35,872
52
votes
6 answers
Why is AC more "dangerous" than DC?
After going through several forums, I became more confused whether it is DC or AC that is more dangerous. In my text book, it is written that the peak value of AC is greater than that of DC, which is why it tends to be dangerous. Some people in…
Four Seasons
- 2,547
52
votes
4 answers
How are magnets held together, and why do they not explode?
Imagine we have a magnet (red side is the north pole, blue side is the south pole), and imagine two ways to split it.
The first way:
When we split it by separating the north pole from the south pole, we see that the two pieces are themselves dipole…
Maximal Ideal
- 8,292
52
votes
3 answers
Are W & Z bosons virtual or not?
W and Z bosons are observed/discovered. But as force carrying bosons they should be virtual particles, unobservable? And also they require to have mass, but if they are virtual they may be off-shell, so are they virtual or not.
user1702
- 553
52
votes
4 answers
Why don't we use infrared light to heat food?
Why don't we use infrared (IR) or even the far IR just to heat food in a microwave oven instead of, of course, the conventional 2.45 GHz microwaves? Don't people call IR heat waves?
ObsessionWithElectricity
- 1,005
52
votes
3 answers
If Newton's third law is true, why can we sink in sand?
Newton's third law of motion states that every action has an equal and opposite reaction. That is the reason we do not sink into the earth, because when our weight exerts a force on the earth it also exerts an equal and opposite force on us.
But…
52
votes
7 answers
Are random errors necessarily Gaussian?
I have seen random errors being defined as those which average to 0 as the number of measurements goes to infinity, and that the error is equally likely to be positive or negative. This only requires a symmetric probability distribution about zero.…
Meep
- 3,959
52
votes
4 answers
Black holes and positive/negative-energy particles
I was reading Brian Greene's "Hidden Reality" and came to the part about Hawking Radiation. Quantum jitters that occur near the event horizon of a black hole, which create both positive-energy particles and negative-energy particles, produce the…
52
votes
1 answer
Why does paper become translucent when smeared with oil but not (so much) with water?
When I smear oil onto a scrap of paper and rub it in, the paper becomes quite translucent; but when I attempt the same with water it doesn't as much. Why?
Mozibur Ullah
- 12,994
52
votes
8 answers
How is the classical twin paradox resolved?
I read a lot about the classical twin paradox recently. What confuses me is that some authors claim that it can be resolved within SRT, others say that you need GRT. Now, what is true (and why)?
vonjd
- 3,701
52
votes
4 answers
Can a tomato pierce a hole in a steel plate if only the tomato is travelling fast enough?
A tomato is travelling very fast towards a 1 cm thick steel plate.
Let's say this happened in a vacuum, so that the air resistance wouldn't rip the tomato apart before it even hit the steel plate.
Obviously the tomato would get destroyed too, but…
Fiksdal
- 656
52
votes
4 answers
What is the mass density distribution of an electron?
I am wondering if the mass density profile $\rho(\vec{r})$ has been characterized for atomic particles such as quarks and electrons. I am currently taking an intro class in quantum mechanics, and I have run this question by several professors. It is…
clevy
- 623
52
votes
4 answers
Has Jaynes's argument against Bell's theorem been debunked?
As a student of theoretical physics I'm well acquainted with the multitude of crackpot ideas attempting to circumvent Bell's theorem regarding local hidden variable theories in quantum physics.
Recently, however, I've been working on my master's…
Timsey
- 1,007
52
votes
5 answers
Why do many people say that virtual particles do not conserve energy?
I've seen this claim made all over the Internet. It's on Wikipedia. It's in John Baez's FAQ on virtual particles, it's in many popular books. I've even seen it mentioned offhand in academic papers. So I assume there must be some truth to it. And…
reductionista
- 1,801
52
votes
8 answers
Books for Condensed Matter after Ashcroft/Mermin
What are some good condensed matter physics books that can fill the gap between Ashcroft & Mermin and research papers? Suggestions for any specialized topics (such as superconductivity, CFT, topological insulators) are welcomed.
leongz
- 3,966